A full-scale nuclear weapon, like the A-bombs dropped on Japan in 1945, could flatten an entire city. By contrast, a radiological bomb -- made up of ordinary chemical explosives saturated with radioactive waste -- wouldn't necessarily significantly damage structures but could spray radioactivity over a large area.

Those substances could range from low-radioactivity debris stolen from hospitals to highly poisonous waste swiped from nuclear power plants or secretly extracted from dump sites in the former Soviet Union or elsewhere.

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One problem: No one has any idea how many could be injured or killed in a radiological attack. That depends on a wide variety of factors, ranging from the amount and type of radioactivity to the means of dispersal -- spread by a bomb? thrown from a plane? poured into a reservoir? sprayed from a truck? -- and weather conditions.

For example, most medical waste from hospitals is only mildly radioactive, whereas much waste from nuclear power plants is significantly "hotter." Some radioactive substances have extremely short half-lives, measured in days, and would quickly disappear; others, such as plutonium, have half-lives of tens of thousands of years.

The health effects of any attack might take years of epidemiological analysis to detect. Given the uncertainty, some say, the most immediate threat of a radiological attack may be not medical but psychological.

The public fear of radiation is so great that a "dirty bomb" attack could trigger mass panic. The hysteria could achieve what radioisotopes might not: destruction of property values and disruption of life and commerce over a large area.

If an attack happens in San Francisco, "people are going to start heading for the bridge -- you'll see car accidents and fatalities, the kind of thing al Qaeda would love to sit back and watch," said Jon Wolfsthal, deputy director of the nonproliferation project at the Carnegie Endowment for International Peace in Washington.

"What terrorists are seeking to do is to prey upon that fear (of low-level radioactivity) that people have, whether or not it's a justified fear," Wolfsthal said.

In that regard, Wolfsthal said, "what would have happened if an airliner that crashed into the World Trade Center had contained radioactive materials? The cleanup would have taken years, not months."

PSYCHOLOGICAL IMPACT

"Look what happened after Sept. 11," said Robert Bartholomew, a Vermont sociologist and author of a study of episodes of mass hysteria, "Little Green Men, Meowing Nuns and Head-Hunting Panics: A Study of Mass Psychogenic Illnesses and Social Delusion."

"There was an influx of (mass hysteria) reports around the world, including the rashes that affected schoolchildren in the U.S.," Bartholomew said. "I would contend that the long-term psychological impact of a real (dirty bomb) detonation could be more damaging than the initial real casualties."

There are no known studies of what the biological effects of a radiological attack in the Bay Area would be. But a recent study of the impact of such an attack on Washington, D.C., suggests dire possibilities.

The study simulated a dirty bomb detonation outside the Smithsonian Air and Space Museum. The exercise found that while the blast itself would cause mass casualties, the radioactive cesium 137 -- a highly poisonous substance with a half-life of 30 years -- spread over one-quarter of Washington could be managed through cleanup. Any health problems, mainly in the form of higher rates of cancer and cataracts, wouldn't be evident for 10 or 20 years.

Planners from police, fire, public health, military agencies and local utilities were confident they could handle the first phase of the disaster caused by a 4,000-pound dynamite bomb set off in a school bus, said Phil Anderson of the Center for Strategic and International Studies, which oversaw the March 21 exercise. But handling public reaction was another matter.

"The participants were extremely comfortable with the conventional aspect of the attack, but were extremely uncomfortable with the radiation aspects," Anderson said.

The public has to be educated not to panic in the face of such an explosion,

he said: "It's the fear factor we've got to focus on. We need to invest resources in public awareness. . . . We can deal with a situation like this simply through education."

In San Francisco, any radiological attack would be handled by the Metropolitan Medical Task Force, a group formed in 1997 that includes Police, Fire and Health department authorities plus state and federal teams.

The possibility of mass hysteria is "one of those things we deal with in any disaster," said Lucien Canton, director of emergency services for San Francisco. After a radiological attack, city officials would try to combat panic partly by providing mental health counseling and feeding "accurate information to the media," he said.

Part of the reason for public fear is the uncertainty over how a dirty-bomb attack would happen.

"The engineering that goes into a dirty bomb is much less sophisticated" than in a nuclear weapon, Wolfsthal said. "You don't need to be a rocket scientist to build a pipe bomb that has radioactive material in it."

POTENTIAL SCENARIOS

Human imagination is the main limit to schemes for spreading radioactive materials over inhabited areas. Wolfstahl said the possibilities included:

-- Placing radioactive materials around an ordinary pipe bomb, then setting it off. If it's dropped from a height, such as a building or airplane, winds would carry the radioactivity a greater distance.

-- Pouring radioactive liquid waste into a street sweeper, which would then spray the liquid around the city.

-- Pouring radioactive liquids into a gas tank, then punching small holes in the tank and driving the car around town. The waste would leak into the streets.

"In Moscow in 1998, there were some Chechen separatists who (left) low- level medical isotopes buried in a park in Moscow," Wolfsthal said. "They contacted the press and said, 'See -- you (the Russian military) should pull out of Chechnya.' "

The stolen radioactivity -- the element americium -- was recovered by Russian authorities. No one was hurt.

Spreading radiation - and terror

An explosion of TNT could be used to spread radioactive materials. Beyond the damage done by the blast itself, the effects of such a crude nuclear attack would probably be more psychological than physical. But so-called "dirty bombs" are a real threat, experts say. Every year, 200 sources of radioactive material for industrial or medical use are listed as stolen or missing in the U.S.

San Francisco scenarios

The two scenarios below assume an explosion occurred at the Transamerica Pyramid building and that winds were traveling east. Weather conditions would dictate the spread of fallout.

DEADLY, BUT LESS LIKELY: 100 pounds of conventional explosives bundled with spent fuel rods from a nuclear reactor.

In the blast zone: Gamma rays would permeate a 300-foot area, exposing victims to about 3,000 rem six times the lethal dosage (one rem equals 100 medical X-rays). Without proper protection, handlers would soon die of overexposure.

Outside the blast zone: Dosages would drop. But prevailing winds could carry a lethal plume up to one kilometer away, about the distance to the Ferry Building. This type of powerful radiation would be capable of penetrating the body. POWERFUL, IN TERMS OF TERROR: 100 pounds of conventional explosives bundled with cobalt-60, which is regularly used in cancer therapy. The radioactive material would be ground, then further pulverized and dispersed by the blast. Ingested particles could cause cancer.

In the blast zone: A maximum dosage of 12 rem would not result in any radiation-related deaths.

Outside the blast zone: Winds could loft and carry particles small enough to be inhaled within the one-kilometer distance. Adverse health effects would be long-term, with risks of sickness rising with the amount of particles ingested.

(X) The base of the Transamerica Pyramid building is about 150 feet wide